The present invention relates to an electronic governor for an internal combustion engine for controlling the rotational speed of the internal combustion engine by controlling a control rack of a fuel injection pump.
In an internal combustion engine, such as a diesel engine where fuel is supplied by a fuel injection pump, the rotational speed (rpm) is controlled by controlling the position of a control rack (fuel injection amount adjustment member).
An example of an electronic governor for controlling the rotational speed by controlling the position of the rack of the fuel injection pump for use in an internal combustion engine is shown in Japanese Patent Application Laying-open No. 171037/1982. In this prior art electronic governor, a rotational speed detection signal obtained from a sensor for detecting the rotational speed of the engine, a rack position detection signal obtained from a sensor detecting the position of the rack of the fuel injection pump, and an accelerator position detection signal obtained from a sensor for detecting the position of the accelerator manipulator are used to calculate the target position of the rack required to obtain the desired rotational speed of the engine as indicated by the accelerator position. A control voltage required for positioning the rack at the target position is generated and is used to drive an actuator for actuating the rack, thereby moving the rack to the target position.
In an internal combustion engine with a prior art governor which determines the manipulated variable from a rack position detection signal, the characteristic curve showing the variation of the rotational speed against the load factor (=actual load/rated load) d droops, i.e., it exhibits a drop characteristic. The degree of drooping is expressed in terms of droop factor D defined by EQU D={(N1/N2)-1}.times.100% (1)
where N1 represents the rotational speed for the load factor of 0%, and
N2 represents the rotational speed for the load factor of 100%.
The term "droop characteristic" is generally used to refer to the characteristic in which the rotational speed decreases with increasing load factor. But, in this specification, the term "droop characteristic" encompasses not only the characteristic of decreasing rotational speed with increasing load factor but also the characteristic of increasing rotational speed with decreasing load factor.
Different droop factors are preferred or required depending on the application of the internal combustion engine. For instance, when the constant speed control by which the rotational speed is kept constant against load variation is to be effected the droop factor needs to be zero.
But with the prior art governor, if the difference between the rotational speed N2 for the load factor of 100% and the rotational speed N1 for the load factor of 0% is very small, the gain of the control system is too large and the engine rotational speed is unstable. For this reason, the droop factor cannot be freely selected.
Moreover, with the prior art system, a rack position sensor for detectin the position of the rack is required for controlling the rotational speed. The system is therefore complicated.